docs: def clean_route_short_names(feed: "Feed") -> "Feed":
      """
      In ``feed.routes``, assign 'n/a' to missing route short names and
      strip whitespace from route short names.
      Then disambiguate each route short name that is duplicated by
      appending '-' and its route ID.
      Return the resulting "Feed".
      """
      feed = feed.copy()
      r = feed.routes
      if r is None:
          return feed
  
      # Fill NaNs and strip whitespace
      r["route_short_name"] = r["route_short_name"].fillna("n/a").str.strip()
  
      # Disambiguate
      def disambiguate(row):
          rsn, rid = row
          return rsn + "-" + rid
  
      r["dup"] = r["route_short_name"].duplicated(keep=False)
      r.loc[r["dup"], "route_short_name"] = r.loc[
          r["dup"], ["route_short_name", "route_id"]
      ].apply(disambiguate, axis=1)
      del r["dup"]
  
      feed.routes = r
      return feed